1. Field of the Invention
This invention relates to a color image display apparatus which is provided with color filters in front of a display face of a monochrome image display unit such as a black-and-white cathode ray tube thereby to display a color image.
2. Description of Related Art
FIG. 1 is a schematic structural diagram of a conventional color image display apparatus using color filters. Referring to FIG. 1, a monochrome image display unit 10 of, for example, a cathode ray tube (CRT) displays a monochromic image onto a display face 10a based on a luminance signal Y, a horizontal synchronizing signal H and a vertical synchronizing signal V. A coloring unit 2 is arranged confronting to the display face 10a of the image display unit 10. The coloring unit 2 is a disk-shaped member as shown in FIG. 2, in which three color filters 2a, 2b, 2c, e.g., red (R), green (G) and blue (B) filters with equal area are successively arranged around a rotary shaft 17. The color filters 2a, 2b, 2c are so set as to sequentially be in front to the display face 10a of the image display unit 10, and driven around the rotary shaft 17 by a rotary driving device, for example, a driving motor 3.
In the vicinity of the rotary shaft 17 is provided a rotation detector 4 which detects the rotational phase of the coloring unit 2. The rotation detector 4 generates a rotational synchronization pulse P of the coloring unit 2 to a control device 6. In consequence, the control device 6 controls the driving motor 3 based on the rotational synchronization pulse P and the vertical synchronizing signal V, with outputting a switching signal S to a signal switching device 5. The signal switching device 5 having inputs of color signals R, G, B corresponding to the respective color filters 2a, 2b, 2c selects and outputs one of the color signals R, G, B for every input of the switching signal S to the image display unit 10.
The operation of the conventional color image display apparatus in the structure as depicted hereinabove will be discussed now. Upon receipt of the vertical synchronizing signal V the frequency of which is demultiplexed to 1/3 and the rotational synchronization pulse P of the coloring unit 2, the control device 6 controls the driving motor 3 so as to maintain a predetermined phase relation, thus eventually controlling the rotation of the coloring unit 2 around the rotary shaft 17 in a direction of an arrow (with reference to FIG. 2). The coloring unit 2 is consequently rotated 1/3 for one field of the input video signal. If the phase relation is set so that the lower half of, for instance, the red color filter 2a is positioned at the uppermost end of the image display unit 10 with the timing of the vertical synchronizing signal V, the coloring unit 2 can be rotated to position the red color filter 2a on the scanning line at all times in accordance with the advancement of the scanning line from the upper to lower portion on the display face 10a of the image display unit 10.
In the meantime, the switching signal S is outputted from the control device 6 to the signal switching unit 5 every time the vertical synchronizing signal V is inputted to the control device 6. As a result, the signal switching unit 5 outputs a signal corresponding to the color of the color filter 2a, 2b or 2c facing the display face 10a of the image display unit 10 every time the switching signal S is input thereto. The control operation will be shown in time sequence in FIGS. 3-6. FIGS. 3(A), 4(A), 5(A) and 6(A) indicate the relationship between the rotational phase of the coloring unit 2 and a scanning line (a) on the display face 10a of the image display unit 10. FIGS. 3(B), 4(B), 5(B) and 6(B) represent the state of the signal switching unit 5 corresponding to the respective FIGS. 3(A), 4(A), 5(A) and 6(A). Although FIGS. 3-6 are related to the case of a red display and the change from a red display to a green display, the same goes true for the green display and blue display. Since the scanning line (a) is enough to be accommodated within the area of each color filter 2a, 2b, 2c, the rotational phase of the coloring unit 2 does not need to be very accurate.
While the coloring unit 2 makes one rotation, the image display unit 10 sequentially displays red (R), green (G) and blue (B) video signals for every field. At this time, also the color filters 2a, 2b, 2c of the coloring unit 2 are sequentially facing to the display face 10a of the image display unit 10, and therefore, a color image in three colors is obtained with the screen of three fields for one rotation of the coloring unit 2.
Because of the above-described structure of the conventional color image display apparatus, video signals for three fields are necessitated to obtain a piece of a color image. In the case of a pattern on components close to primary colors, only one field of the three fields is displayed, thus generating undesired flicker.
Meanwhile, there has been proposed to use a color CRT of a shadow mask type or to combine images of three monochromic CRTs by means of a dichroic mirror etc. The former method is disadvantageous in that the resolution is decreased to 1/3 of a black-and-white CRT since three colors are decomposed when displayed on the same CRT. According to the latter method, since it is necessary to adjust the position of the three CRTs with high accuracy, manufacturing of the display apparatus is difficult and moreover, manufacturing costs are high because of the need of a highly accurate dichroic mirror in addition to the three CRTs.